The present invention relates to communications methods and apparatus, and more particularly, to methods and apparatus for allocating resources in wireless communications.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (mobile terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modem digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carrier frequencies are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Hybrid TDMA/CDMA systems have been proposed. Examples of such systems are described in U.S. Pat. No. 5,790,549 to Dent (issued Aug. 4, 1998), U.S. Pat. No. 5,566,168 to Dent (issued Oct. 15, 1996), U.S. Pat. 5,539,730 to Dent (issued Jul. 23, 1996), U.S. Pat. No. 5,481,533 to Honig et al. (issued Jan. 2, 1996) and in the xe2x80x9cDraft PCS2000 Standard (PN-3390),xe2x80x9d offered to the Joint Technical Committee on Wireless Access JCT(Air) by the Technical Ad Hoc Group (TAG) One (Omnipoint Corporation), Oct. 31, 1994.
Recently, wireless communications systems have seen a rapidly increasing demand for services other than transmission, including, for example, text messaging and multimedia services such as internet access, video and the like. Each of these services typically has different performance requirements. As it would generally be impractical to provide separate wireless infrastructures for each of these services, there is a need for wireless communications apparatus and methods whereby multiple services with differing performance requirements can utilize a common infrastructure in an effective and efficient manner.
According to the present invention, a terminal is assigned to an entire time slot or to a subchannel of a time slot defined by a spreading code based on one or more communications constraints associated with the terminal, e.g., based on a performance requirement, such as information rate or bit error rate, associated with the terminal, and a signal reception condition, such a signal to noise ratio, under which the terminal is operating. According to an aspect of the present invention, this assignment is performed such that system resources, such as the number of available time slots, available bandwidth, available spreading codes, and transmit power are optimized. In this manner, multiple services having varying performance requirements can be efficiently served by a common wireless infrastructure.
In particular, according to an aspect of the present invention, a wireless communications system including at least one base station operative to communicate on a plurality of carrier frequencies in repetitive time slots defined thereon is operated by assigning an entire time slot or a spreading-code defined subchannel of a time slot to a terminal based on a communications constraint associated with the terminal. The communications constraint may include a performance requirement, such as an information rate or an error rate, and a signal reception condition, such as signal to noise ratio.
According to an embodiment of the present invention, a single terminal is identified and a communications constraint associated with the single terminal is identified. A plurality of other terminals is also identified, and respective communications constraints associated with respective ones of the plurality of terminals are determined. A first time slot is exclusively assigned to the single terminal based on the communications constraint associated with the single terminal. A second time slot is assigned to the plurality of terminals based on the communications constraints associated with the plurality of terminals. The system communicates with the single terminal using the first time slot, and communicates with respective ones of the plurality of terminals in the second time slot using respective subchannels, wherein respective ones of the plurality of channels are encoded according to respective spreading codes, which may be direct-sequence spreading codes or scrambling masks.
The system may communicate with the single terminal at a first information rate, and may communicate with a terminal of the plurality of terminals at a second information rate lower than the first information rate. Multiple subchannels of the second time slot may be assigned to one of the terminals of the plurality of terminals. According to another embodiment of the present invention, the second time slot is assigned to a high-penetration channel, such that the subchannels represent subchannels of the high-penetration channel.
According to another aspect of the present invention, methods of operating a wireless communications system are provided. The wireless communications system is operative to communicate with terminals using a plurality of time slots on a plurality of carrier frequencies, to communicate on respective subchannels on a time slot according to respective spreading codes and to communicate over a channel with a variable coding rate and a variable bandwidth. The system determines a communications constraint associated with a terminal. The system assigns a time slot, a spreading code, a coding rate and a bandwidth to the terminal based on the determined communications constraint.
The communications constraint may include a performance requirement and a signal reception condition. The performance requirement may include at least one of an information rate or an error rate. The signal reception condition may include a signal to noise ratio. The system may assign a time slot, a spreading code, a coding rate and a bandwidth to the terminal such that at least one of an available bandwidth, a number of available time slots, and a number of available spreading codes is optimized.
According to another aspect of the present invention, a wireless communications system operative to communicate on a plurality of carrier frequencies in repetitive time slots defined thereon includes means for determining a communications constraint associated with a terminal. Means, responsive to the means for determining a communications constraint, are provided for assigning one of an entire time slot or a spreading-code defined subchannel of a time slot to the terminal based on the determined communications constraint. Means are also provided for communicating with the terminal on the assigned entire time slot or subchannel.
According to another aspect of the present invention, a wireless communications system includes means for determining a communications constraint associated with a terminal, and means, responsive to the means for determining a communications constraint, for assigning a time slot, a spreading code, a coding rate and a bandwidth to the terminal based on the determined communications constraint. Preferably, the assigning means assigns a time slot, a spreading code, a coding rate and a bandwidth to the terminal such that at least one of an available bandwidth, a number of available time slots, a number of available spreading codes, or a transmit power is optimized.
According to yet another aspect of the present invention, a wireless communications system includes a base station operative to communicate using a plurality of time slots on a plurality of carrier frequencies, to communicate on respective subchannels on a time slot according to respective spreading codes and to communicate over a channel with a variable coding rate and a variable bandwidth. A resource allocator is operatively associated with the base station and operative to determine a communications constraint associated with a terminal and to assign a time slot, a spreading code, a coding rate and a bandwidth to the terminal based on the determined communications constraint. The base station is responsive to the resource allocator to communicate with the terminal according to the assigned time slot, spreading code, coding rate and bandwidth. Preferably, the resource allocator is operative to assign a time slot, a spreading code, a coding rate and a bandwidth to the terminal such that at least one of an available bandwidth, a number of available time slots, a number of available spreading codes, or a transmit power is optimized.
According to yet another aspect of the present invention, a terminal for communicating with a wireless communications system includes means for informing the system of a communications constraint associated with the terminal, and means for communicating with the system using a time slot, a spreading code, a coding rate and a bandwidth assigned to the terminal by the system based on the determined communications constraint. According to an embodiment of the present invention, the terminal includes means for communicating with the system using an entire time slot or a subchannel of a time slot defined by a spreading code.